The countenance of the second mate brightened, for he prided himself not a little on his vast and varied stores of knowledge, and nothing pleased him so much as to be questioned, particularly on knotty subjects. "Hem! yes, Mivins, I can tell 'ee that. Ye must know that before fresh water can freeze on the surface the whole volume of it must be cooled down to 40 degrees, and _salt_ water must be cooled down to 45 degrees. Noo, frost requires to be very long continued and very sharp indeed before it can cool the deep sea from the top to the bottom, and until it is so cooled it canna freeze." "Oh!" remarked Mivins, who only half understood the meaning of the explanation, "'ow very _h_odd. But can you tell me, Mr. Saunders, 'ow it is that them 'ere _h_icebergs is made? Them's wot I don't comprehend no'ow." "Ay," replied Saunders, "there has been many a wiser head than yours, puzzled for a long time about icebergs. But if ye'll use yer eyes you'll see how they are formed. Do you see the high cliffs yonder away to the nor'-east? Weel, there are great masses o' ice that have been formed against them by the melting and freezing of the snows of many years. When these become too heavy to stick to the cliffs, they tumble into the sea and float away as icebergs. But the biggest bergs come from the foot of glaciers. You know what glaciers are, Mivins?" "No, sir, I don't." The second mate sighed. "They are immense accumulations of ice, Mivins, that have been formed by the freezings and meltings of the snows of hundreds of years. They cover the mountains of Norway and Switzerland, and many other places in this world, for miles and miles in extent, and sometimes they flow down and fill up whole valleys. I once saw one in Norway that filled up a valley eight miles long, two miles broad, and seven or eight' hundred feet deep; and that was only a wee bit of it, for I was told by men who had travelled over it that it covered the mountains of the interior, and made them a level field of ice, with a surface like rough, hard snow, for more than twenty miles in extent." "You don't say so, sir!" said Mivins in surprise. "And don't they _never_ melt?" "No, never. What they lose in summer they more than gain in winter. Moreover, they are always in motion; but they move so slow that you may look at them ever so closely and so long, you'll not be able to observe the motion--just like the hour hand of a watch--but we know it by observing the changes from year to year. There are immense glaciers here in the Arctic Regions, and the lumps which they are constantly shedding off into the sea are the icebergs that one sees and hears so much about."